Techniques for joining lined pipelines

ABSTRACT

A connector for lined pipelines includes a tube having opposed male interface elements extending inwardly from respective ends of the tube. One or more circumferential permeable chokes project radially from each male interface element. The chokes minimise flow of oxidising fluid from the bore into the micro-annulus between the liner and the pipe while maximising flow of fluid from the micro-annulus into the bore in the event of catastrophic pressure drop in the bore. To maintain gaps between the tube ends and the pipe liners for fluid flow, shoulder formations extend circumferentially around the tube. The connector may be used in a joint arrangement where each liner includes a body, an end of lesser thickness and greater bore than the body that terminates short of an end of the pipe, an inner step between the body and the end, and an outer step between the end and the pipe.

This invention relates to techniques for joining lined pipelines as usedin the oil and gas industry for production, for wafer injection or forother purposes. The invention is particularly concerned with a connectorfor bridging across a weld between abutting lined pipes. The inventionis also concerned with methods for making, connecting and welding suchlined pipes.

Corrosion protection is a key issue for pipelines used in the oil andgas industry, which are usually made of carbon steel to reduce cost overoften great lengths. Liners of composites or plastics, for example, highdensity polyethylene (HDPE) have been used for many years to mitigateinternal corrosion of such pipelines, as an alternative to moreexpensive liners of common-resistant alloys (CRA) such as Inconel 625(trade mark). Plastics or composite liners also aid insulation of thepipeline.

For brevity, plastics or composite liners will be referred tocollectively in this specification as plastics Briers unless the contextrequires otherwise.

Whilst effective, plastics liners suffer from problems in practice. Onesuch problem is permeation of fluids, During operation, liquids and/orgases present in the fluid flowing through the pipeline tend to passthrough (or, via any gaps, around) the liner to enter the micro-annulusbetween the liner and the surrounding pipe. Transfer of fluid into themicro-annulus will continue until the pressure in the micro-annulusmatches that of the fluid flowing through the bore of the pipeline.Then, any rapid reduction of pressure in the bore will lead to anoverpressure in the micro-annulus that may not equalise quickly and maytherefore cause the liner to collapse inwardly. This is a particularproblem for water injection service where sudden depressurisation isknown to have occurred.

Another problem of plastics liners is that they create difficulties whenforming joins between lengths of lined pipe. The heat of conventionalbutt welding between the steel pipe lengths may damage the lining.Whilst it is possible to join the lengths of pipe by other means such asthreaded or flanged connections, such connections are often impracticalin the context of subsea operations where, for example, a pipeline mayneed to be spooled onto a reel or launched from a vessel using an S-layor J-lay technique.

There is a need for an easy-to-install liner connector that provides foreffective dermal protection and effective equalisation of pressurebetween the micro-annulus and the bore to avoid liner collapse, butwithout exposing the Inner surface of the pipe to excessive corrosion.There is also a need to enable lengths of steel pipe with plasticsliners to be joined by welding while maintaining a substantiallycontinuous corrosion-resistant internal surface between them.

The connector of the invention satisfies those needs, providing aconnector that bridges the pipe weld and engages with the liner in theencompassing pipes while surviving the thermal load of the weldingprocess. The connector of the invention also permits equalisation ofpressure in the micro-annulus while sufficiently isolating the innerwall surface of a steel pipe from bulk of the product carried by thepipeline to reduce corrosion of that surface.

Efforts have been made previously by others to address those needs. Forexample, U.S. Pat. No. 5,988,891 to Coflexip discloses a pipe jointcomprising welded pup pieces lined with CRA. End pieces of HDPE arefusion-welded to respective liners in the abutting pipes and carry CRAinserts that are seal-welded to the CRA lining of the pup pieces. A gapis left between the end pieces to permit butt welding of the pipe endswithout damage to the liners, although an optional transition sleeve maybe provided to fill the gap. However, there is no provision in U.S. Pat.No. 5,988,691 for venting the micro-annulus between the liner and thepipe.

WO 02/33288 to Boreas discloses a lined pipeline having vent devicesextending through the liner to allow gas to flow from the micro-annulusto the bore. There is no discussion in WO 02/33298 of a Joint betweenabutting lengths of lined pipe. In contrast, WO 2004/011840, also toBoreas, proposes a pipe liner connector for connecting adjacent lengthsof pipe liner in a Pressure Balanced Joint (PBJ). An objective of thePBJ is to control exposure of the pipe wall to corrosive agents,specifically by preventing mass transport of bore fluids to the pipewall so as to minimise replenishment of corrosion-promoting agents,especially oxygen or nutrients for sulphate-reducing bacteria. To thisend, o-rings seal the pipe liner connector to the liners of the abuttingpipe lengths, Vents (as sold under the trade mark ‘Linavent’) extendthrough the wall of the pipe liner connector, with the aim of allowinggas to flow from the micro-annulus to the bore for equalisation. Acentral heat-shielding cylindrical layer lies below the interfacebetween abutting pipe lengths to enable welding without damaging thepipe liner connector. This allows regular carbon steel welding for allpasses, including the root pass.

Boreas' solution in WO 2004/011840 is unworkable in practice its sealingo-rings do not have sufficient elasticity to cope well with ovality ormisalignment of the pipes. Pipes are manufactured slightly oval in crosssection and the ovality is modified by bending and straighteningprocesses involved in pipeline deployment from a pipe-laying ship.Additional ovalisation of 4 mm on a 12″ OD (305 mm) pipe of 22.2 mm wallthickness is typical and it will often occur in a different radialdirection to the natural ovality of the pipe. Also, the pipe linerconnector of WO 2004/011840 must be held in place by locking rings,which adds to the cost, complexity and process time of creating a joint.

The connector of WO 2004/011840 has other drawbacks. For example, itcannot accommodate significant movement or ‘creep’ of the liner withrespect to the pipe. Creep will occur after the liner is Installed andis also experienced during bending and straightening operations on aninstallation vessel when pipelaying. Also, the connector of WO2004/011840 is not optimised to reduce turbulence in the fluid flowingthrough the bore of the pipeline.

Another problem with the connector of WO 2004/011840 is that even if theo-ring seals are effective (which, due to ovality, they probably willnot be), a substantial volume of liquid may enter and travel along themicro-annulus between the liner and the pipe. That liquid will passthrough the narrow bores (typically 3 mm diameter) of the ‘Linavent’vents leading from the bore through the wall of the connector to theannulus around the connector and from there to the micro-annulus betweenthe liner and the pipe. Post-deployment, the compressive loads betweenthe liner and the pipe are nearly equal on diametrically-opposed sidesof the pipe wall and the lowest compressive load between the liner andthe pipe is on the major axis of the pipeline between those opposedsides. This provides a path for conveying liquid along themicro-annulus. In practice, flow may be generated along themicro-annulus between the liner and the pipe and the annulus between theconnector and the liner.

Even at low differential pressures of say 0.5 bar, the micro-annulus ofa lined pipe has been observed to hold an estimated 1.6 litres of liquidover a typical 11 m pipe ‘joint’. Such a volume of liquid in contactwith the inner wall of the pipe may promote corrosion, especially ifthere is any significant interchange of liquid between the bore themicro-annulus that with refresh the oxygen or other corrosive agents inthat volume of liquid. The connector of WO 2004/011840 recognises thisproblem and attempts to reduce interchange as the ‘Linavent’ vents arebaffled with a sintered frit. However the bigger problem is that onceliquid has entered the micro-annulus, it presents a risk of collapsingthe liner and the connector if there is a rapid drop of pressure of theproduct flowing along the bore of the pipe. The ‘Linavent’ vents are tooconstricted to allow rapid equalisation of pressure between themicro-annulus and the bore of the pipe where a substantial volume ofliquid is present in the micro-annulus.

There remains a need for a simple and economical joint that allowseffective equalisation while being suitable for use with offshore-weldedplastics-lined pipe joints used in S-lay and J-lay techniques.

It is against; this background that the present invention has been made.

From one aspect, the invention resides in a connector for linedpipelines, the connector comprising: a tube having opposed ends, thetube defining opposed male interface elements extending inwardly fromrespective ends of the tube; and at least one circumferential permeablechoke projecting radially from each male interface element to controlfluid flow around the tube in use.

The Inventive concept also encompasses a pipeline joint arrangementbetween lined pipes abutting end-to-end, wherein the joint comprises aconnector of the invention as defined above, bridging spaced-apartliners of the pipes; and each liner comprises: a body section; an endsection of lesser thickness and greater bore than the body section; andan inner step disposed between the body section and the end section; andwherein: a respective male interface element of the connector isreceived telescopically by the end section of each liner, with mutualclearance defining an annular channel between the male interface elementof the connector and the end section of the liner; a respective end ofthe tube opposes the inner step of each liner; and at least onepermeable choke extends radially from each male interface element to theopposed end section of each liner to control fluid flow in therespective channels in use of the pipeline.

The inventive concept extends to a pipeline comprising a plurality ofconnectors of the invention as defined above or a plurality of jointarrangements of the invention as defined above.

The choke permits pressure equalisation through the channel whilerestricting interchange of liquid between the bore of the pipeline andthe micro-annulus between the liner and the pipe. Whilst each maleinterface element may have only one choke, it is also possible for thereto be a plurality of chokes on each male interface element, the chokesbeing disposed sequentially relative to a fluid flow direction throughthe chokes. Preferably, the tube of the connector has a continuous wallthat is uninterrupted by openings between its ends. Thus, the chokealone controls flow of fluid around the connector.

The connector of the invention preferably further comprises shoulderformations that project radially from the exterior of the tube and arelocated inward of respective male interface elements of the connector. Ashoulder formation may, for example, be defined by a band extendingcircumferentially around the tube. This strengthens and protects thetube and an insulator layer positioned around the tube. It is alsopossible to remove the connector from a pipe by pulling on the shoulderformation.

More specifically, spaced-apart bands may define respective shoulderformations, in which case the insulator layer is suitably disposedbetween the bands for protection. To this end, it is preferred that thebands project radially beyond the thickness of the insulator layer. Theinsulator layer can be made from any suitable compliant or segmentedinsulation material, and must not interfere with the root of the weld;it may, for example, comprise a microgel insulator supported by abacking tape. The bands are suitably disposed symmetrically each side ofa central plane that bisects the tube and that is oriented orthogonallyto its central longitudinal axis.

Where the end section of the finer terminates short of an end of thepipe, the shoulder formation of the connector suitably opposes an outerstep of the finer disposed between the end section and the pipe. Thislimits axial movement of the connector with respect to the liner.Preferably, the distance between the shoulder formation and the nearestend of the tube is less than the length of the opposed end section ofthe liner, and the distance between the inner steps of the liners islonger than the tube of the connector. This ensures that a gap ismaintained between at least one end of the tube and the opposed innerstep of the liner, which gap communicates between a channel and the boreof the tube.

It is preferred that the shoulder formation defines an overall width ofthe connector that is less than the internal diameter of the pipes inthe region between their abutting ends and the end sections of theirliners. This maintains clearance to reduce heat transmission to theconnector during welding. The outer diameter of the shoulder formationshould clear the root of the weld to enable a simple axial pull in orderto recover the connector.

It is also preferred that the internal tubular surface of the connectoris substantially aligned with the bore of a lined pipe at the bodysection. This reduces turbulence of fluid flowing along the pipe andfacilitates pigging. To this end, the difference in liner thicknessbetween the body section and the end section of the liner is preferablysubstantially equal to the wall thickness of the tube plus the clearancebetween the male interface element of the connector and the end sectionof the liner.

The liners of the pipes are preferably oversized in outer diameter withrespect to the internal diameter of the pipes, rolled-down and pultrudedinto position within the pipes, without locking rings.

Radiusing or chamfering is preferably applied to various edges andcorners of the connector or the liner for stress relief, to easeassembly of a joint and/or to reduce turbulence in use. Radiusing orchamfering may for example be applied to least one of: an inner radialend edge of the tube; an outer radial end edge of the tube; an innerradial edge of the outer step; and an Inner radial edge of the innerstep. However, over-radiusing of the chamfers would be detrimental tothe stability of the connector at a desired position with respect to theliner.

The inventive concept also embraces the choke itself. The choke of theinvention comprises a permeable mass such as a porous foam and/or abarrier web penetrated by openings to confer permeability. The choke maybe received in a circumferential groove extending around the maleInterface element and suitably bears resiliency against the end sectionof the liner with radial force. Nevertheless, it is preferred that thechoke is collapsible away from the liner over a threshold differentialpressure applied across the choke.

Where employed, the porous mass is suitably retained in a holdercomprising a base web and at least one retaining wall upstanding fromthe base web. Where employed, the barrier web may be inclined relativeto a base web of the choke or to the male element the connector. Forexample, the barrier web may be inclined outwardly from the male elementtoward the adjacent end of the tube.

More generally, the choke may be asymmetric in section. It may have acomposite structure comprising first and second components of differentstiffness and/or permeability, which components are preferably disposedsequentially relative to a fluid flow direction through the choke. Thus,the choke may have asymmetric response to pressure within the channel toeach side of the choke.

The inventive concept also encompasses a method of assembling a pipelinejoint, comprising: providing a first lined pipe whose liner comprises: abody section; and an end section of lesser thickness and greater borethan the body section: inserting into the end of the first pipe aconnector of the invention, such that one male interface element of theconnector is received telescopically by the end section of the linerwith the choke extending radially between the male interface element andthe end section, while leaving the opposed male interface elementprotruding from the end of the first pipe; bringing a second lined pipeinto end-to-end abutting relation with the first pipe, the liner of thesecond pipe having a corresponding body section and end section suchthat the opposed mate interface element of the connecter is receivedtelescopically by the end section of the liner of the second pipe, withthe choke of the opposed male interface element extending radiallybetween that male interface element and that end section; and joiningthe pipes where they abut end-to-end.

Where the connector has a shoulder formation and the liner of the firstpipe terminates short of an end of the pipe, insertion of the connectorinto the first pipe is preferably limited by the shoulder formation ofthe connector bearing against an outer step of the liner disposedbetween the end section and the pipe. More preferably, the shoulderformation of the connector bears against the outer step of the linerbefore the end of the tube of the connector contacts an inner step ofthe liner disposed between the body section and the end section.

The pipes are suitably joined by welding, in which case cooling ispreferably applied internally within the tube of the connector. Suchcooling may, for example, be provided by air blown radially against theInternal surface of the connector tube. It is also possible to cool thepipe externally after a welding station or between welding stations. Thetemperature of the connector may be controlled by monitoring theexternal pipe temperature around the weld.

An alignment tool may be attached to either pipe, the alignment toolcomprising a plurality of alignment blocks angularly spaced around andoverlapping an end of the pipe. Then, the other pipe may be insertedbetween the overlapping parts of the alignment blocks before bringingthe second lined pipe info end-to-end abutting relation with the firstpipe.

This specification primarily describes pipelines for water injection,which experience a particularly high rate of corrosion due to dissolvedoxygen in the filtered sea water flowing within. The invention willtherefore be described with particular reference to steel pipes linedwith HDPE for carrying sea water, but it should be understood that thematerials described may be varied as necessary to meet the requirementsof other fluids. Thus, the inventive concept has wider application andmay have benefit in handling hydrocarbons and other corrosive fluids.

It should be noted that in a broad sense, the invention is not limitedto welded pipelines because other techniques such as adhesives or amechanical connection may be used for joining lengths of pipe. Also theinvention is not limited to a contact/interference connection betweenthe connector and the pipe; for example, a form of thermal welding couldbe used, enabling the connector to be left in place in case theintegrity of the pipe weld/connection is breached. It should also benoted that the broadest concept of the invention is not limited to steelpipelines, as some features of the connector may have benefit inpipelines made of other materials such as composites.

In order that the invention may be more readily understood, referencewill now be made, by way of example, to the accompanying drawings inwhich:

FIG. 1 is a schematic part-sectioned side view of a connector inaccordance with the invention, in use engaged with two lined pipes;

FIG. 2 is longitudinally-sectioned scale view of a connector and pipescorresponding to FIG. 1:

FIG. 3 is an enlarged detail view of an interface part highlighted inFIG. 2;

FIG. 4 corresponds to FIG. 3 but shows a variant having double chokes ateach end of the connector;

FIG. 5 is a perspective view of an optional cooling head for Internalair cooling of the connector during welding;

FIG. 6 is a schematic side view showing the optional use of a tool forinserting the connector into a pipe and for removing the connector fromthe pipe;

FIG. 7 is a schematic plan view of the insertion/removal tool of FIG. 6,in use;

FIG. 8 is a schematic cross-sectional view of the insertion/removal toolof FIGS. 6 and 7, in use;

FIG. 9 is a side view of an optional external alignment clamp being usedto align successive lengths of pipe;

FIG. 10 is a cross-sectional view of the alignment clamp of FIG. 9, inuse;

FIG. 11 is a schematic cross-sectional view of a pipe and connectorshowing the effect of ovality of the pipe;

FIGS. 12a to 12e are schematic cross-sectional views showing variouschoke options;

FIGS. 13a and 13b are schematic cross-sectional views showing a furtherchoke option in use, with different clearances between a lined pipe anda connector; and

FIGS. 14a and 14b are schematic cross-sectional views that correspond toFIGS. 13a and 13b but show another choke option.

Where the drawings or the following description include dimensions,those dimensions are given for the purpose of context and to aidunderstanding: they do not limit the scope of the invention.

Referring firstly to FIGS. 1 to 3 of the drawings, two lined carbonsteel pipes 20 abut end-to-end and are joined by a circumferential weld22. Only one side of each pipe 20 is shown in the schematic view of FIG.1 but both sides of the pipes 20 are shown in the scale drawing of FIG.2. Each pipe 20 comprises a pultruded liner 24 of HDPE 100. Thewelded-together pipes 20 enclose a generally tubular connector 26 thatextends between their liners 24. A section of pipe 20 or pipe joint istypically 11 m in length. The pipe 20 is nominally of circular crosssection but will generally have some ovality as will be explained.

The liners 24 have interface formations machined into their opposed endsto mate with inverse interface formations provided on the opposed endsof the connector 26. Specifically, the liners 24 terminate short of anend of the pipe 20 and each liner 24 has a stepped cross-sectioncomprising a full-thickness body portion 28 at which the bore of thepipe 20 is relatively narrow and a reduced-thickness end portion 30 atwhich the bore of the pipe is relatively wide. In cross-section, the endportion 30 of the liner 24 is arranged concentrically with respect tothe body portion 28 of the liner 24 and with respect to the pipe 20.

Each liner 24 therefore has a stepped profile in longitudinal section,defining two annular steps 32, 34. An inner or blocking step 32 definesthe Junction between the body portion 28 and the and portion 30 of theliner 24. The inner step 32 shown in the simplified view of FIG. 1 liesin a plane orthogonal to the central longitudinal axis of the pipe 20.The inner step 32 and the end portion 30 of the liner 24 are created byboring the liner 24 from the end of the pipe 20. Conversely, an outer orinitiation step 34 defines the junction between the end portion 30 ofthe liner and the section of unlined pipe 20 extending from there to theend of the pipe 20. The outer step 34 is created by machining the liner24 inside the pipe 20 alter post-pultrusion shrinkage of the liner 24has stabilised, as will be explained below.

The following description will assume that the internal surface of thepipe 20 extending beyond the liner 24 to the end of the pipe 20 is bareand exposed. However, before lining, the interior of the pipe 20 couldbe spray-coated in a coated zone extending inwardly for say 50 mm to1000 mm from a weld preparation zone at the end of the pipe 20.

The radially inner edge 36 of the outer step 34 of each liner 24 isshown chamfered in FIG. 1 to ease insertion of the connector 26 into apipe 20. It is also possible, though not shown in the simplified view ofFIG. 1, to chamfer the radially inner edge of the inner step 32 of eachliner 24 to reduce turbulence. Chamfering involves additional expenseand is optional. It is also possible to radius the radially inner edgeof the inner step 32 as best shown in the enlarged detail view of FIG.3.

Known lining techniques pull a loose-fitting liner into a pipe and lockthe liner in place with an Inconel locking ring of wedge-shapedcross-section or a swaged (i.e. corrugated) Inconel cylinder. TheApplicant's trials show that pultruding an oversized liner 24 info apipe 20 (for example a 305 mm OD liner into a 276 mm ID pipe) ispossible in one roll-down stage. The liner 24 can then expandelastically within the pipe 20 to lock it into place, although linercreep producing relative axial movement between the liner 24 and thepipe 20 remains a factor as will be explained. The liner 24 reducesaccess of oxygenated sea water to the inner wall of the pipe 20, hencereducing corrosion of the pipe 20.

It is possible to pultrude a larger liner 24 (for example 315 mm OD)into the same pipe 20 but this may require two roll-down stages, whichadds time to the process and would provide no advantage as plasticdeformation offsets the elastic memory of the liner 24 to some extent.Roll-down in stages is only useful where a large enough pipe 20 or asmall enough liner 24 to allow a single roll-down operation are notavailable when required.

The oversized liner 24 creates considerable friction with the wall ofthe pipe 20, quantified as 9TeF (9×10⁴N) per axial metre pulling forceand greater than 40TeF (4×10⁵N) per axial metre pushing force in a 12″(305 mm) OD pipe. In practice, it is the greater pushing force that mustbe overcome because there is no mechanism to engage the liner 24 andpull it in the normal operating environment within the pipe 20.

The high axial friction force means that metal locking rings are notrequired for the liner 24. However, this does not mean that the liner 24will remain fixed relative to the pipe 20 once if has been installed.Within two hairs of pultrusion into a pipe 20, the ends of the liner24—which would generally be left proud of the pipe 20—shrink bycentimetres and will creep by further centimetres over the next fewweeks. It is therefore advisable not to machine the ends of the liner 24until a period of at least two weeks, and preferably at least fourweeks, have elapsed after pultrusion. However that period could beshortened by heat treatment, involving blowing hot air or passing hotwater down the lined pipe 20.

The liner 24 will creep by centimetres more at each end due to stressrelief during the bending and straightening processes involved indeployment of a pipeline from a pipe-laying vessel. The length of thestepped end portion of the liner 24 must be sufficient to compensate forliner creep; the connector 26 illustrated in FIGS. 1 to 3 must alsoallow for this with the length of its inverse interlace step.

The connector 26 aims to mitigate corrosion with a continuousvariable-length barrier that handles axial shrinkage and creep of theliner 24 while providing continuity of lining across the pipe weld 22between two adjoining lengths of pipe 20. The connector 26 must have thethermal robustness necessary to survive the pipe welding process. Theconnector 26 may be made of a choice of composite materials andplastics, with the option to be made of a different material to theliner 24. However, the exemplary connector 26 illustrated and describedin this specification is made of the same material as the liner 24, Thethermal insulation properties of the connector 26 are comparable tothose of the liner 24.

The connector 26 comprises an elongate tube 38 that is machined ormoulded from a polymer, HDPE 100. Internally, the tube 38 is plain andparallel-walled apart from turbulence-reducing internal chamfers 40around the ends of the tube 38. Externally, the tube 38 has variousfeatures disposed symmetrically about a central plane 42 orthogonal tothe central longitudinal axis of the tube 38, that plane 42 beingsubstantially aligned with the weld 22 when the connector 26 is in situas show in FIG. 2, Those external features are:

-   -   ‘shoulder’ formations defined by circumferential integral        hoop-reinforcement bands 44 extending around the tube 38, the        bands 44 being parallel to and spaced from each other about the        central plane 42 of the connector 26 to define an insulator        recess 46 between them;    -   an insulator strip 48 laid on the outside of the tube 38,        extending across the insulator recess 46 between the bands 44        and thus being aligned with the weld 22 when the connector 26 is        in situ;    -   identical circumferential chokes 50 projecting radially from        circumferential grooves 52 extending continuously around the        tube 38 near its ends, for example about 30 mm in from the outer        ends of the connector 26; and    -   external lead-in chamfers 54 around the ends of the tube 38.

Whilst grooves 52 are shown to locate the chokes 50 in this example, itwould also be possible to locate a choke 50 on the connector 26 with alocking ring.

In general, 15° chamfering of all corners that might cause turbulence orimpede insertion of the connector 26 into, and extraction of theconnector 26 from, the lined pipe 20 is preferred. Similarly, corners ofthe connector 26 and the liner 24 are preferably radiused for thepurpose of stress relief. For example, the bands 44 shown in thesimplified schematic view of FIG. 1 have shoulders 56 that extend fromthe tube in planes orthogonal to the central longitudinal axis of thetube 38. However, in practice as shown in the scale views of FIGS. 2 and3, substantial radii should be applied to the base of the bands 44 forstress relief because the central part of the tube 38 between the bands44 behaves as a cylindrical diaphragm in use. Similarly, wall thicknessshould be maximised around the central pad of the tube 38 to reducestrain on the ends of the insulator recess 46

The length of the insulator recess 46 of the connector 26, correspondingto the width of the insulator strip 48, is approximately 200 mm axiallyfor a 12″ (305 mm) OD pipe 20. The insulator strip 48 used for testingpurposes comprises a microgel insulator approximately 2 mm thick, forexample an Aerogel or Pyrogel (trade marks) supplied by Aspen or aNanogel (trade mark) supplied by Cabot Corporation. This insulator isbacked by Argweld (trade mark) backing tape supplied by HuntingdonFusion Techniques Limited, both being held around the central part ofthe tube 38 with aluminium tape of, for example, 50 mm to 75 mm inwidth. The aluminium tape may include a central thread marker to alignthe insulated centre of the connector 26 with the root of the weld 22.Other insulator materials and constructions are possible, it isemphasised that the dimensions given above are merely by way of example;they are likely to be minimum dimensions but this remains to feeconfirmed by testing.

The chokes 50 comprise an elastomer and various embodiments of them willbe described in more detail later, with reference to FIGS. 12a to 12e,13a, 13b, 14a and 14 b.

FIGS. 1 and 2 show that the ends of the tube 38 lie between the opposedinner steps 32 of the liners 24. To recap, those inner steps 32 arebetween the full-thickness body portions 28 of the liners 24 and theirreduced-thickness end portions 30. The external diameter of the tube 38is slightly less than the internal diameter of the end portions 30 ofthe liners 24, the resulting clearance defining annular channels 58between the tube 38 and the respective end portions 30. Nominally, thedepth of each channel 58 defined by the clearance between the tube 38and the respective end portions 30 of the liners 24 is typically of theorder of 1 mm. However the clearance could be less, potentially zero atsome locations, if the connector 26 is inserted with a pneumatic orhydraulic tool.

Each channel 58 forms part of a fluid flow path extending from the bore60 of the pipeline to the micro-annuli 62 between the liners 24 and thepipes 20, via the exterior of the connector tube 38 and the annulus 64defined by the gap between the liners 24 where the pipes 20 adjacent theweld 22 are exposed. A gap 66 of circa 10 mm between each end of thetube 38 and the opposed inner step 32 accommodates tolerances in theassembly and communicates with a respective channel 58 to complete thefluid flow path.

The fluid flow paths equalise pressure of the annulus 64 and themicro-annuli 62 with the bore 60 of the pipeline and so prevent theliner 24 or the connector 26 collapsing due to differential pressure inuse of the pipeline. Equalisation will work at any orientation of roll,pitch or yaw of the pipeline. This process of pressure transfer isrequired to prevent the connector 26 and the liner 24 from deforming ifoverpressure in the annulus 64 and the micro-annuli 62 exceedsapproximately 0.5 bar. In tests in which equalisation was prevented,deflection of the insulator recess 46 between the circumferential bands44 of the connector 26 was noted at an overpressure 0.5 bar and thisdeveloped into severe bulging at 0.9 bar. Complete collapse occurred atan overpressure of approximately 1.5 bar.

Fluid flow through each channel 58 is restricted by a choke 50 thatbears resiliency and radially outwardly from the tube 38 against theopposed end portion 30 of the liner 24 to restrict—but not prevent—fluidflow along the fluid flow paths as will be explained. Pressureequalisation is required before a damaging overpressure condition arisesbut only limited two-way flow is allowed between the micro-annuli 62,the annulus 64 and the bore 60 of the pipeline. Thus, the chokes 50 areporous and permeable to allow pressure equalisation but they preventlarge-volume flow at low differential pressure, although they may bedesigned to collapse to permit high-volume flow at high differentialpressure as will be explained with reference to FIGS. 12a to 12 e.

Having regard to the depth of the channels 58 between the tube 38 andthe end portions 30 of the liners 24, the wall thickness of the tube 38is correspondingly less than the height of the inner steps 32. Thismeans that the internal surface of the tube 38 substantially aligns withthe internal surfaces of the body portions 28 of the liners 24.Alignment between the internal surfaces of the connector 26 and thelined pipes 20 reduces turbulence within the pipeline in use, and alsofacilitates pigging of the pipeline. However, alignment need not beexact, as pigs can tolerate a minor misalignment of the bores of theliner 24 and the connector 26. Indeed, a standard pig can be used,helped by the connector 26 being approximately the same internaldiameter as the lined pipe section, the narrow gaps 66 between theconnector 26 and the lined pipe section, and the chamfered, taperingentry and exit of the connector 26.

The pair of bands 44 extending around the tube 38 provide some hoopstress reinforcement to the tube 38. In situ, they lie between, andusually slightly spaced from, the opposed spaced ends of the liners 24defined by their outer steps 34. The external diameter of the bands 44is greater than the internal diameter of the end portions 30 of theliners 24, but is less than the internal diameter of the pipes 20exposed between the liners 24 to either side of the weld 22.

There is no need for a band 44 to touch the exposed section of pipe 20once the connector 26 is fully inserted into a pipe 20, as the bands 44play no part at that stage in centralising the connector 26 within thepipe 20. The bands 44 may, however, contribute to coarse centralising onInitial insertion of the connector 26 into the pipe 20. They also helpto protect the insulator strip 48 and the chokes 50 from sharp edges,dust, debris and mill scale that typically arise from weld profilepreparation of steel pipes.

Indeed, some radial clearance between the bands 44 and the pipes 20 ispreferred so that there is no risk of obstruction with the pipe wall onheavily ovalised pipe, and also to hinder heat transfer from the pipes20 to the connector 26 during welding. Also, the bands 44 may jam underthe root of the weld 22 if they are too closely matched to the internaldiameter of the pipes 20, although this is only a problem if theconnector 26 ever has to be extracted from the pipeline during repairs.

A key purpose of the bands 44 is to restrict relative axial movementbetween the connector 26 and the liners 24. If there is substantialrelative axial movement between the connector 26 and a liner 24, ashoulder 56 defined by one of the bands 44 will bear against the outerstep 34 of the liner 24 to prevent further relative axial movement inthat direction. Otherwise, such movement could close the gap 66 betweenthe end of the tube 38 and the inner step 32 of the liner 24 and henceblock the fluid flow path. However even in that event, the gap 66 at theother end of the tube 38 would be expected to remain open to maintain analternative fluid flow path to assure equalisation.

Further redundancy is assured by the provision of two chokes 50 on theconnector 26 in the event of blockage at one end of the connector 26,for example due to silt. The connector 26 need only have a single choke50 at each end although it could have additional chokes. For example,double chokes 50 at each end of the connector 26, as shown in thevariant of FIG. 4, would provide redundancy but would halve thepotential flow for a given permeability of choke, unless thepermeability of each choke 50 was doubled. Also, the interface stepwhere projecting portions of the tube 38 extend axially beyond the bands44 to the ends of the connector 26 may have to be extended toaccommodate additional chokes 50, although this is not shown in FIG. 4.

It will be apparent from FIGS. 1 to 4 how the interface formations ofthe opposed liners 24 comprise female interface elements that matetelescopically with male interface elements defined by the inverseinterface formations of the connector 26. This allows for liner creepwhile the bands 44 prevent excessive axial shuttle movement of theconnector 26 between the liners 24. For this purpose, there is asubstantial male-female overlap between the connector 26 and each liner24 at the interface, specifically at the projecting portions of the tube38 where the tube 38 extends axially outwardly beyond the bands 44 tothe chokes 50 and from there to the ends of the connector 26. Themajority of each projecting portion of the tube 38 lies within acorrespondingly-elongated end portion of the liner 24. This allows forextensive relative axial movement between the connector 26 and theliners 24 while maintaining channels 58 between the tube 38 and theliners 24 and keeping the chokes 50 in resilient sliding contact withthe liners 24. Thus, the connector 26 compensates for manufacturingovality of the pipes 20, which in turn distorts the liner 24; it alsocompensates for radial distortion of the pipes 20 during bending andstraightening in the lay process, and for relative axial movement of theliners 24 with respect to the pipes 20.

The length of the overlap between the connector 26 and the liners 24must take account of axial shrinkage and creep of the liners 24throughout the lifetime of the pipeline. Consequently a minimum overlapat each end of the connector 26 of 160 mm (200 mm maximum) is envisagedfor pipes 20 of 12″ (305 mm) diameter. The length of this overlapdepends on the diameter of the liner 24, its thickness and the relativediameters of the pipe 20 and the liner 24.

Thus, the liner interface mates with the inverse Interface of theconnector 26 to provide interfacing surfaces for the chokes 50. Theinterface step centres the connector 26 in the lined pipes 20. The bands44 of the interface also provide two end stops to restrict shuttlemotion of the connector 26 while still enabling the connector 26 tomaintain contact with the liners 24 via the chokes 50.

In use when assembling a pipeline, the connector 26 is inserted into theend of a pipe 20 prepared as shown in FIGS. 1 and 2, with half of theconnector 26 protruding from the pipe 20. It is proposed that the bestplace to insert the connector 26 is when the pipe 20 is in a ‘readyrack’ after a bevelling station. This maintains environmental protectionof the connector 26, which may remain packaged up to that point, andallows a period of about twenty minutes to unpack and load the connector26 and to dispose of its packaging. Then, a second similarly-preparedpipe 20 is brought together into end-to-end abutment with the first pipe20 while surrounding and locating the connector 26 as shown in FIGS. 1and 2. Finally the pipes 20 are welded together after any necessaryfurther preparation.

During weld preparation and during the welding process itself, theinsulator strip 48 protects the connector 26 beneath from radiant heatfrom the weld 22. The connector 26 can survive manual and automaticwelding at any angle of pipe 20. The insulator strip 48 must be belowthe root of the weld 22 and is preferably centralised with respect tothe weld root. However if the insulator strip 48 is offset with respectto the weld root, the edge of the insulator strip 48 should preferablyoverlap by at least 5 mm beyond the root gap.

To protect the HDPE material of the connector 26 below the insulatorstrip 48 in close proximity to the weld 22 and also beyond theprotection of the insulator strip 48, the root of the weld 22 should nottouch the outer surface of the insulator strip 48: a standoff distanceof >3 mm would assist the thermal survival of the HDPE. To this end, theconnector 26 should not touch the inner wall of the pipes 20 as notedabove, not even at the circumferential bands 44. The liners 24 take thefirst of the thermal shock of welding by conduction from the pipes 20.

During early welding trials involving the invention, thermocouples (notshown) were placed at various points on the outside surface of theconnector 26 and at approximately corresponding points on the Inside ofthe connector 26. In addition, temperature readings were taken atcorresponding positions axially spaced from the weld 22 on the outersurface of the pipes 20 so that the temperature at each point in thiscross-sectional matrix could be monitored and compared. After severaltest welds, it could be concluded that—for a particular lined pipeconfiguration—the connector 26 will not be thermally damaged by heatsaturation if a temperature limit of, say, no more than 280 Celsius isobserved on the outer surface of the pipes 20 more than 25 mm from theweld toe. Thus, it is possible to model the thermal response of thesystem and then to infer from a simple external temperature measurementof a pipe 20 that the temperature experienced by the connector 26 iswithin desired limits, without having to measure the temperature of theconnector 26 directly. If the threshold external temperature is indanger of being breached, key welding variables may be changedaccordingly. It is also possible to apply and/or to modify internalcooling within the pipes 20 to control the temperature of the connector26.

It is possible to avoid cooling the connector 26 during the weldingprocess with careful selection of welding variables. However, internalcooling of the connector 26 may be advantageous as this allows fasterwelding while keeping the temperature of the HDPE material well belowits softening temperature. Initial trials suggest that greater internalcooling is appropriate during manual TIG welding than during auto MIGwelding.

FIG. 5 of the drawings shows a cooling head 68 that may, if desired, beused for internal air cooling of the connector during welding. Thecooling head takes air at nominally 4 to 8 bar (i.e. suitable for rigservice air) through a pneumatic quick-connect coupling 70 and blaststhat air laterally through nozzles 72 facing radially outwardly, againstthe interior of the connector 26 in approximate alignment with the weld22. Spaced parallel centraliser plates 74 of the cooling head 68 fitclosely within the connector 26 to support the nozzles 72 around thecentral longitudinal axis of the connector 26.

In tests, internal air cooling using the cooling bead 68 in conjunctionwith auto MIG and manual TIG welding was effective in reducing thetemperature of the connector 26 in all cases and configurations. Theresults show that with limited heat management optimisation to maintainthe external temperature of the pipes 20 more than 25 mm from the weldtoe at less than 280 Celsius, the connector 26 will survive weldingwithout distortion. Monitoring the region of the weld toe with athermocouple or non-contact IR thermometer to ensure a maximum externalpipe temperature of 280 Celsius can be used to infer the maximumtemperature being experienced by the connector 26 and hence to the guidethe whole welding and/or cooling process. Of course, this guidetemperature is merely an example and can be determined for anypipe/liner configuration.

Internal air cooling has been found to bring the temperature of teeconnector 26 down from 110 Celsius to below 50 Celsius, thus more thanhalving the temperature experienced by the HDPE material. In this way,it has been found possible to weld a 12″ (305 mm) OD pipe with auto MIGwelding while experiencing a temperature of under 50 Celsius at themidpoint on the connector 26 with cooling in twenty minutes when all thewelding passes are undertaken serially atone station.

Moving on now to FIGS. 6 to 8 of the drawings, these show a poweredinsertion tool 76 to grip and insert the connector 26 into a lined pipe20 and subsequently to extract the connector 26 from the pipe 20 shouldthere be a problem. The insertion tool 76 comprises a frame 78 that maybe suspended by a lifting point 80, claws 82 pivotally attached to theframe 78 by respective claw axles 84 parallel to the centrallongitudinal axis of the pipe 20, and a pneumatic or hydraulic claw ram86 acting on the claws 82 to close them about the pipe 20. The claws 82grip the exterior of the pipe 20 to lock the frame 78 relative to thepipe 20.

The frame 78 supports parallel slider tubes 88 that, in turn, support acursor 90 that bridges the slider tubes 88 and is movable along theslider tubes 88 in a direction parallel to the central longitudinal axisof the pipe 20. A pneumatic or hydraulic ram 92 acts between the frame78 and the cursor 90 to drive the movement of the cursor 00 along theslider tubes 88.

Arms 84 depend from the cursor 90 to embrace an end of the connector 26.When inserting the connector 26, the arms 94 bear against a ‘top hat’flanged insert 96 of HDPE that is positioned between the arms 94 and theconnector 26 to prevent damage to the connector 26. The ram 92 pulls thecursor 90 and hence the connector 26 toward the pipe 20 to insert theconnector 26 up to its mid point in the end of the pipe 20.

The same tool 76 can be used in a reverse stroke if it is necessary toextract the connector 26 from the pipe 20. In that case, movable orremovable fingers 98 on the arms 94 to each side of the cursor 90 arehooked behind a circumferential shoulder band 44 of the connector 26.The ram 92 can then push the cursor 90 and hence the connector 26 awayfrom the pipe 20 to extract the connector 26.

Another option to extract the connector 26 is simply to use a winch (notshown). For example, a hole may be tin lied throughdiametrically-opposed walls of the connector 26, whereupon a pin may bepushed through the holes and attached to a winch that pulls theconnector 26 out of the pipe 20. Obviously this technique would lead tothe connector 26 being scrapped but this would generally be the fate ofa connector that has to be removed in any event

FIGS. 9 and 10 of the drawings show an external alignment clamp 100, asthe inside of the lined pipe 20 is too soft to apply large radialalignment forces to the pipe 20 with a traditional internal alignmentclamp. An external alignment clamp could be incorporated into theinsertion/extraction tool 78 of FIGS. 6 to 8 but the basic tool shown inFIGS. 9 and 10 is simpler and yet effective.

The alignment clamp 100 comprises a circumferential hoop 102 dividedinto two semi-circular sections hinged together, that can be openedabout the hinge 104 to admit a second pipe 20′ and then closed aroundthe second pipe 20′, close to its end, and held together by a latch 106diametrically opposed to the hinge 104.

Three alignment blocks 108 are equi-angularly spaced around the hoop102. They present inner faces to the second pipe 20′ that lie parallelto its central longitudinal axis and that overlap its end. Eachalignment block 108 has a tapered front 110 inclined outwardly andforwardly to assist alignment of the second pipe 20′ with a first pipe20 as shown in FIG. 9.

The effect of ovality of a pipe 20 is apparent in the exaggeratedschematic view of FIG. 11 of the drawings. Here, the pipe 20 isflattened from top to bottom as shown. This results in two annuli thatvary in section depending on the angular position around the centrallongitudinal axis of the pipe 20. The first annulus is the micro-annulus62 between the pipe 20 and the liner 24, which is squeezed to a minimumat the top and bottom of the cross-section and flares to a maximum atthe sides. Similarly, the liner 24 is flattened from top to bottom ofthe cross section as shown and this affects the second annulus, namelythe clearance defining the channel 68 between the projecting maleportion of the tube 38 and the surrounding female end portion of theliner 24

Specifically, the clearance is reduced to a minimum (practically zero)at the top and bottom quadrants and Increases to a maximum (potentially4 mm for a 12″ (305 mm) pipe) at the side quadrants, but the choke 50contracts radially at top and bottom and expands radially at the sidesto accommodate these differences. Meanwhile, an effective fluid flowpath is maintained through the choke 50 around the sides of the tube 38to ensure equalisation, even if the path is blocked at the top andbottom.

The aggregate cross-sectional area of the channel 58 between the tube 38of the connector 26 and the surrounding and portion 30 of the liner 24varies with ovalisation. However it can be shown that even in an extremecase where the channel 58 is 4 mm wide at its widest and 0 mm wide atits narrowest, the aggregate cross-sectional area of the channel 58increases by a factor of less than 1.5 over a non-ovalised channel 58whose width is consistently 0.5 mm around the circumference. Thisensures reasonably consistent behaviour of a choke 50 disposed in thechannel 58.

O-ring seals between the telescoping parts of the liner 24 and theconnector 26 would not work in this situation as they do not have thedynamic range necessary to compensate for ovalisation. By way ofexplanation, o-ring seals as proposed in the aforementioned WO2004/011840, which are typically of nitrile rubber, have acompressibility of no more than about 25%. This restricts their dynamicradial range. The larger the section of the o-rings, the greater theirdynamic radial range in absolute terms. However the section of theo-ring is limited by practical considerations, for example by the axialforce needed to insert the connector into a lined pipe. O-rings ofgreater than circa 6.3 mm cross-section are considered impractical andtheir dynamic radial range will be barely 1.5 mm, hence rendering themincapable of handling the degree of ovality typically experienced inreal pipelines.

Similarly, if would not be possible to rely solely upon a close slidingor interference fit between the telescoping parts of the liner 24 andthe connector 26 to control the fluid flow around the connector 26. Thatsolution would suffer from the risk that ovalisation would disrupt thefit, with clearance being too small in some places to allow the parts tomove past one another or too large in other pieces to restrict the fluidflow as desired.

Thus, turning now to FIGS. 12a to 12e of the drawings, these showvarious possible choke arrangements 50A to 50E respectively inaccordance with the invention. Those variants demonstrate bow to controlfluid flow between the pipe bore 60 and the micro-annulus 62 withisobaric, symmetrical or asymmetrical reciprocal pressure transmissionacross a choke. They also show how the permeability and pressureresponse of a choke may be modified with different dispositions andcombinations of porous materials and barriers.

In normal circumstances, a choke must restrict movement of corrosivefluid such as oxygenated sea water to minimise the corrosive effect ofthat fluid on the inner wall of the steel pipe 20. By restricting thatmovement, oxygen depletion upon initial corrosive oxidation of the pipewall will reduce the corrosive effect of the fluid and hence slowfurther corrosion. In other words, minimal flow between the wall of thepipe 20 and the pipe bore 60 will reduce the rate at which corrosiveoxidiser is replenished and hence will reduce the corrosive effect ofthat oxidiser. Yet, it is also important that if pressure in the pipebore 60 drops suddenly, then liquid in the micro-annulus 62 between theliner 24 and the pipe 20 and in the annulus 64 encircling the connector26 can flow quickly into the bore 60 to equalise pressure and so toprevent collapse of the liner 24 or the connector 26.

In all cases, the resilient radial extensibility of the choke—itsdynamic radial range—allows for the varying clearance between the liner24 and the connector 26, which—as FIG. 11 shows—varies with angularposition about the central longitudinal axis of the pipe 20 inaccordance with the degree of pipe ovalisation before and during the layprocess, and may indeed continue to vary with creep of the liner 24 andmovement of the connector 26 in use of the pipeline. The choke musttherefore continue to compensate for such changes in geometry throughoutthe working life of the pipeline. The permeability of the choke shouldbe set such that the minimum acceptable flow is set by the lowestexposed cross-sectional area of the choke and the maximum allowable flowshould be set by the largest exposed cross-sectional area of the chokeencountered upon maximum ovalisation of the pipe 20.

An aim of the choke variants 50A to 50E shown in FIGS. 12a to 12erespectively is therefore to provide a large dynamic radial range tocompensate for the ovality seen in FIG. 11. Such chokes may, forexample, provide a dynamic radial range of approximately 6 mm outsidethe confines of its groove 52, which may in turn be approximately 4 mmdeep. Thus, the choke will require radial compressibility ofapproximately 60% while continuing to bear radially outwardly againstthe liner 24, which is possible with careful selection of porosity andsubstrate hardness.

FIGS. 12a, 12b and 12c show choke variants 50A to 50C respectively inwhich porous, permeable choke material 112 such m resilient foam issupported in a holder 114 comprising a base web 116 and resilientretaining walls 118 extending upwardly and integrally from the base web116. The choke 50A shown in FIG. 12a has a triangular-section length ofchoke material 112 whose apex ridge 120 faces away from the base web 118and whose base is retained by inwardly-inclined retaining walls 118.Similar retaining walls 118 are a feature of the base web of the choke50B shown in FIG. 12b , the distinction here being that the length ofchoke material 112 protruding from the holder 114 does not taper to anapex ridge but instead is parallel-sided and terminates in a roundedupper surface 122. The length of choke material 112 protruding from theholder of the choke 50C shown in FIG. 12c is similarly parallel-sidedand terminates in a rounded upper surface 122, but in that case theretaining walls of the base web 116 are mirror-image C-sections.

The basis of the further choke variants 50D and 50E shown in FIGS. 12dand 12e respectively is an acutely-angled ‘V’ shaped profile comprisinga resilient inclined barrier web 124 upstanding from and integral with abase web 116. The barrier web 124 is penetrated by openings such as cutsor perforations so as to act as a choke. An array of holes 126 is shownin the barrier web 124 of FIGS. 12d and 12e ; slits are possiblealternative openings.

In the choke 50E of FIG. 12e , the barrier web 124 partially encloses atriangular-section length of porous, permeable choke material 112 suchas resilient foam disposed between the barrier web 124 and the base web116. The choke 50E of FIG. 12e is therefore of composite constructionhaving a relatively stiff permeable spine that supports, and is disposedin series with, the relatively flexible choke material 112. The relativestiffness of the barrier web 124 and the choke material 112 may bevaried to obtain desired properties of deflection, pressure response andresilience. The relative permeability of the barrier web 124 and thechoke material 112 may also be varied to tailor the permeability of thechoke 50E as a whole, for example by providing a less permeable barrierweb 124 that makes the choke 50E more restrictive than a choke ofsimilar shape and thickness made from foam alone.

The choke variants 50A to 50E shown in FIGS. 12a to 12e sharecharacteristics that are highly beneficial in the context of linedpipelines. For example, the cross-sectional area of a circumferentialchoke 50 is massively greater than the cross-sectional area of a‘Linavent’ vent such as is proposed in WO 2004/011840. This provides amuch wider, less constricted and less easily-blocked flow path for therapid escape under high pressure of any liquid accumulated in themicro-annulus 62 between the liner 24 and the pipe 20 and in the annulus64 around the connector 26. The ability to equalise pressure rapidly isenhanced by the ability of the choke 50 to deform under highdifferential pressure, effectively to collapse away from the liner 24above a threshold differential pressure to allow bypass flow of liquidaround the choke 50, through the resulting gap between the liner 24 andthe choke 50. Thus, the material and the structure of the choke 50confers a low modulus of elasticity on the choke 50. Yet, at lowdifferential pressure, the choke 5 bears reliably against the liner 24,regardless of ovalisation, to permit minimal controlled flow that willequalise minor or low-frequency pressure fluctuations withoutundermining corrosion protection.

The inclination of the barrier web 124 evident in the choke variants 50Dand 50E of FIGS. 12d and 12e also lends directional qualities to thechoke, which may be tailored provided that the choke remains flexibleenough to be attached to the connector 26 and then to allow theconnector 26 to be inserted into the lined pipe 20. So, a barrier web124 inclined to the left as shown. In the chokes 50D and 50E of FIGS.12d and 12e has an asymmetric response to differential pressure acrossthe choke, presenting a different response to pressure exerted from theright side of the choke, as shown, than to the same pressure exertedfrom the left side of the choke, as shown. In particular, if themicro-annulus 62 between the liner 24 and the pipe 20 is disposed to theright of the choke 50D and 50E as shown in FIGS. 12d and 12e , thenliquid will enter the micro-annulus 62 less freely than it can exit themicro-annulus 62. It will be noted in this respect that the inclinationof the barrier web 124 as shown is such as to enable substantial flow ofliquid from the right, where at high differential pressure the barrierweb 124 may bend away from the liner 24 to create a bypass path.Conversely, the inclination of the barrier web 124 is such as to resistsubstantial flow of liquid from the left. In that case, the barrier web124 would be forced against the liner 24 to ensure that liquid cannotbypass the choke 50D and 50E even at high differential pressure but mustinstead flow through the choke 50D and 50E. The rate at which liquidflows through the choke 50D and 50E will then depend on the permeabilityof the choke SOD and SOB and the differential pressure to which thechoke 50D and 50E is subjected.

FIGS. 13a and 13b and FIGS. 14a and 14b illustrate further chokevariants 50F and 50G respectively. Like numerals are used for likeparts. Each choke 50F and 50G is shown in situ in a groove 52 around atube 38 of a connector 26. Again, these chokes 50F and 50G have a largedynamic radial range that allows for varying clearance between the liner24 and the connector 26, for example with angular position about thecentral longitudinal axis of the pipe 20 as arises with ovality. Andagain, various measures are possible to tailor the response of thechokes 50F and 50G to differential pressure, such as adjusting stiffnessand permeability.

FIGS. 13a and 13b and FIGS. 14a and 14b Illustrate the extremes of thedynamic radial range hut are not to scale. FIGS. 13a and 14a show anear-maximum clearance between the liner 24 of a pipe 20 and theconnector 26, with the chokes 50F and 50G biased radially outwardlyagainst the liner 24 in a resiliently-extended state. Conversely, FIGS.13b and 14b show a near-minimum clearance between the liner 24 and theconnector 26, with the chokes 50F and 50G collapsed underradially-inward pressure from the liner 24 against their resilient bias.

Referring specifically now to FIGS. 13a and 13b and particularly to thechoke 50F when in the extended state shown in FIG. 13a , the choke 50Fin that extended state has an acutely-angled ‘V’ shaped profilecomprising a resilient inclined barrier web 124 upstanding from andintegral with a base web 118. The barrier web 124 and the base web 116are both generally planar and there is an acute angle between them, suchthat the barrier web 124 extends over the base web 116. The base web 118is slightly narrower than the width of the groove 52 and is offset toone side of the groove 52, leaving a gap 128 at one end.

An apex portion 130 at the radially-outer free edge of the barrier web124 is enlarged and rounded in cross section where it bears against theliner 24. The convex outer surface of the apex portion 130 allows thebarrier web 124 to deflect more easily under-differential pressure torneither side than the sharper-edged choke variants 50D or 50E of FIGS.12d and 12 e.

As the clearance between the liner 24 and the connector 26 reduces, theangle between the barrier web 124 and the base web 116 narrows due toangular movement of the barrier web 124 about the junction 132 betweenthem. Rolling and sliding movement of the apex portion 130 with respectto the liner 24 accommodates this changing geometry while the barrierweb 124 continues to bear radially outwardly against the liner 24.

Eventually as the clearance between the liner 24 and the connector 26approaches a minimum, the choke 50F assumes the collapsed state shown inFIG. 13b . Here, the barrier web 124 has pivoted radially inwardly aboutits junction 132 with the base web 116 to lie substantially parallel tothe base web 116. The enlarged apex portion 130 of the barrier web 124is then accommodated within the gap 128 left by the offset position ofthe base web 116 in the groove 52.

The barrier web 124 is penetrated by a line of holes 126 so as to act asa choke. Some or all of the holes 126 could be filled by a porousmaterial (not shown) that restricts flow while maintaining somepermeability. The permeability of the porous material may be adjusted orvaried, and could vary from hole to hole. Again, slits are possiblealternative openings.

The choke 50F shown in FIGS. 13a and 13b also has an array ofmicrogrooves 134 extending generally in the flow direction. In thisexample, the microgrooves 134 extend over and around the apex portion130 of the barrier web 124 to create a bypass path around the barrierweb 124 when the choke 50F is in the extended state. This optionalfeature uses the microgrooves 134 to tailor the permeability of thechoke 50F; the number and width of the microgrooves may of course bevaried to achieve whatever characteristics may be desired. As themicrogrooves 134 extend around the convex-curved apex portion 130 of thebarrier web 124, they maintain an effective bypass path when the choke50F is in the extended state, irrespective of the clearance between theliner 24 and the connector 26 and hence the inclination of the barrierweb 124.

In the example shown in FIGS. 13a and 13b , the microgrooves 134 spanthe width of the barrier web 124 from the junction 132 to the apexportion 130, so as to maintain an effective bypass path when the choke50F is in the collapsed state as shown in FIG. 13b with minimalclearance between the liner 24 and the connector 26.

Microgrooves 134 may extend either around the apex portion 130 or acrossthe barrier web 124, or as shown in FIGS. 13a and 13b they may extendboth around the apex portion 130 and across the barrier web 124.

Referring finally to the choke 50G shown in FIGS. 14a and 14b , thiscomprises a resilient moulding 136 of arcuate cross-section that issubstantially symmetrical about a central radial plane. Thecross-section of the moulding 136 is part-elliptical in this examplewhen in the extended state shown in FIG. 14a , with mirror-image webs138 extending radially outwardly from the base of the groove 52 in theconnector 26 and converging to join at a central radially-outer apexportion 140. The width of the moulding 136 defined between theradially-inner bases of the webs 138 is offset to one side of the groove52, leaving a gap 128 at one end.

The apex portion 140 of the moulding 136 is rounded in cross-sectionwhere it bears resiliently against the inner surface of the liner 24.The convex outer surface of the apex portion 140 helps the moulding 136to deflect under differential pressure from either side and also toslide with respect to the liner 24 if clearance varies between the liner24 and the connector 26.

As the clearance between the liner 24 and the connector 26 reduces, thecross-section of the moulding 136 flattens as the angle between the webs138 widens due to angular movement about the apex portion 140 betweenthem. The width of the moulding 136 between the radially-inner bases ofthe webs 138 increases as the radial thickness of the moulding 136decreases.

Eventually as the clearance between the liner 24 and the connector 26approaches a minimum, the choke 50G assumes the collapsed state shown inFIGS. 14b . Here, the apex portion 140 lies flattened between the webs138 and the moulding 136 has spread across the gap 128 to fill the widthof the groove 52.

Each web 138 of the moulding 136 is penetrated by a line of holes 126 soas to act as a choke. Some or all of the holes 126 could be filled by aporous material (not shown) that restricts flow while maintaining somepermeability. The permeability of the porous material may be adjusted orvaried, and could vary from hole to hole or from web to web. Again,slits are possible alternative openings.

The choke 50G shown in FIGS. 14a and 14b also has an array ofmicrogrooves 134 extending generally in the flow direction. In thisexample, the microgrooves 134 extend over and around the apex portion140 of the moulding 136 to create a bypass path around the moulding 136when the choke 50G is in the extended state. Again, this optionalfeature uses the microgrooves 134 to tailor the permeability of thechoke 50G and the number and width of the microgrooves may foe varied toachieve desired characteristics. As the microgrooves 134 extend aroundthe convex-curved apex portion 140 of the moulding 136, they maintain aneffective bypass path when the choke 50G is in the extender state,irrespective of the clearance between the liner 24 and the connector 26.

In the example shown in FIGS. 14a and 14b and as best shown in FIG. 14b, the microgrooves 134 extend from the apex portion 140 onto the webs136 of the moulding 136. The microgrooves 134 thereby maintain aneffective bypass path when the choke 50G is in the collapsed state asshown in FIG. 14b , with minimal clearance between the liner 24 and theconnector 26.

The choke variants 50A to 50G shown in FIGS. 12a to 14b sharecharacteristics that are highly beneficial in the context of linedpipelines. For example, the cross-sectional area of a circumferentialchoke 50 is massively greater than the cross-sectional area of a‘Linavent’ vent such as is proposed in WO 2004/011840. This provides amuch wider, less constricted and less easily-blocked flow path for therapid escape under high pressure of any liquid accumulated in themicro-annulus 62 between the liner 24 and the pipe 20 and in the annulus64 around the connector 26. The ability to equalise pressure rapidly isenhanced by the ability of the choke 50 to deform under highdifferential pressure, effectively to collapse away from the liner 24above a threshold differential pressure to allow bypass How of liquidaround the choke 50, through the resulting gap between the liner 24 andthe choke 50. Thus, the material and the structure of the choke 50confers a low modulus of elasticity on the choke 50. Yet, at lowdifferential pressure, the choke 50 bears reliably against the liner 24,regardless of ovalisation, to permit minimal controlled flow that willequalise minor or low-frequency pressure fluctuations withoutundermining corrosion protection.

The inclination of the barrier web 124 evident in the choke variants50D, 50E and 50F of FIGS. 12d, 12e, 13a and 13b also lends directionalqualities to the choke, which may be tailored provided that the chokeremains flexible enough to be attached to the connector 26 and then toallow the connector 26 to be inserted into the lined pipe 20. So, abarrier web 124 inclined to the left as shown in the choke variants 50D,50E end 50F of FIGS. 12d, 12e, 13a and 13b has an asymmetric response todifferential pressure across the choke, presenting a different responseto pressure exerted from the right side of the choke, as shown, than tothe same pressure exerted from the left side of the choke, as shown. Inparticular, if the micro-annulus 62 between the liner 24 and the pipe 20is disposed to the right of the choke variants 50D, 50E and 50F as shownin FIGS. 12d, 12e, 13a and 13b , then liquid will enter themicro-annulus 62 less freely than it can exit the micro-annulus 62. Itwill be noted in this respect that the indication of the barrier web 124as shown is such as to enable substantial flow of liquid from the right,where at high differential pressure the barrier web 124 may bend awayfrom the liner 24 to create a bypass path. Conversely, the inclinationof the barrier web 124 is such as to resist substantial flow of liquidfrom the left. In that case, the barrier web 124 would be forced againstthe liner 24 to ensure that liquid cannot bypass the choke 50D, 50E and50F even at high differential pressure but must instead flow through thechoke 50D, 50E and 50F. The rate at which liquid flows through the choke50D, 50E and 50F will then depend on the permeability of the choke 50D,50E and 50F and the differential pressure to which the choke 50D, 50Eand 50F is subjected.

Double parallel chokes 50 at each end of the connector have beenmentioned briefly above and illustrated in FIG. 4 of the drawings. Thisprovides some redundancy, for example if one choke 50 perishes and socan be bypassed, but in that case the effective increase in permeabilitywould result in an increase in flow for a given pressure differentialacross the chokes 50. This should be considered because if thepermeability is too low then there is a risk of the chokes 50 becomingclogged over lime but if the permeability is too high, then this mayencourage corrosion. However, on balance, this is still better thanrelying upon an o-ring, which either seals or does not; and for reasonsof ovality—and, over time, degradation—will not seal properly and sowill not contribute effectively to corrosion protection by reliablyrestricting the flow of fluids between the bore 60 and the internalsurface of the pipe 20.

The choke variants 50A to 50G of the invention demonstrate how a chokemay be tailored or tuned in various ways, such as permeability,asymmetry and/or the provision of microgrooves. Such measures may beused separately or in conjunction. Additionally, if asymmetric chokesare used at each end of the connector, their directions need not besymmetrical. Also, where there are dual chokes, the chokes at each endof the connector could be in opposition.

In summary, therefore, the invention provides a connector for linedpipes providing continuity of lining, an effective corrosion barrier andchemical barrier, and thermal protection of the corrosion harder. Theliner continuity system of the invention can join pipes made of anymaterial with a connector whose material matches or complements theliner material. Effective pressure equalisation is ensured between thebore and the micro-annulus, with optional provision for thresholddifferential pressure and tunable pressure and flow rates. The connectormaintains thermal insulation properties of the liner with respect to theproduct carried by the pipeline and the temperature of the environmentin which the pipeline operates. It compensates for axial creep of theliner after lining and after pipeline deployment, and also compensatesfor radial distortion and radial creep of the liner.

The invention claimed is:
 1. A subsea lined pipeline for use in the oiland gas industry, the pipeline comprising a connector bridgingspaced-apart liners of the pipeline, the connector comprising: a tubehaving opposed ends, the tube defining opposed male interface elementsextending inwardly from respective ends of the tube, wherein arespective male interface element of the tube is received telescopicallyby a corresponding opposed section of one of the liners of the pipelineto define an annular channel between that male interface element and theopposed section of the liner; and an annular choke extendingcircumferentially around, and projecting radially outwardly from, eachmale interface element to the opposed section of the liner, wherein thechoke comprises a porous mass such as foam and/or a barrier webpenetrated by openings, the choke has permeability to control fluid flowaround the connector in the respective annular channel by permittingfluid flow through the choke in use of the pipeline.
 2. The pipeline ofclaim 1, wherein the barrier web is penetrated by openings, wherein atleast some of the openings are filled with a permeable material.
 3. Thepipeline of claim 1, wherein the barrier web is inclined relative to abase web of the choke.
 4. The pipeline of claim 1, wherein the choke hasa porous mass retained in a holder comprising a base web and at leastone retaining wall upstanding from the base web.
 5. The pipeline ofclaim 1, wherein the choke is asymmetric in section.
 6. The pipeline ofclaim 1, wherein the choke is a composite structure comprising first andsecond components of different stiffness and/or permeability.
 7. Thepipeline of claim 6, wherein the components are disposed sequentiallyrelative to a fluid flow direction through the choke.
 8. The pipeline ofclaim 1, wherein the choke has a liner-engaging surface with bypassformations.
 9. The pipeline of claim 8, wherein the bypass formationsare grooves extending around the choke.
 10. The pipeline of claim 1,wherein the choke has a radially-outer apex formation that is convex incross section.
 11. The pipeline of claim 1, wherein, in longitudinalsection, a radially outer face of the choke is inclined outwardlytowards the respective male interface element.